Elsevier

Chemical Engineering Journal

Volume 197, 15 July 2012, Pages 283-288
Chemical Engineering Journal

One-step hydrothermal preparation strategy for nanostructured WO3/Bi2WO6 heterojunction with high visible light photocatalytic activity

https://doi.org/10.1016/j.cej.2012.05.032Get rights and content

Abstract

The novel heterojunction WO3/Bi2WO6 photocatalysts were successfully prepared by a facile hydrothermal preparation strategy. The as-prepared sample consists of WO3 nanoparticles growing on the primary Bi2WO6 nanoplatelets. The different morphology of the WO3/Bi2WO6 can be obtained by adjusting the amount of nitric acid and Na2WO4·2H2O. Because of the visible light response and appropriate band edges of Bi2WO6 and WO3, the WO3/Bi2WO6 hierarchical heterojunction exhibits high photocatalytic activity over that of the pure Bi2WO6 and WO3 towards the decomposition of rhodamine B (RhB) under visible light irradiation. This can be ascribed to the interconnected interface of WO3 and Bi2WO6 heterostructure, which leads to the high transfer rate of photoinduced charge carriers. More importantly, the hierarchical WO3/Bi2WO6 heterojunction is not only highly stable but also very easy to be separated by simple filtration for reuse.

Highlights

► The hierarchical heterojunction WO3/Bi2WO6 was prepared by hydrothermal reaction. ► The heterojunction WO3/Bi2WO6 shows high and steady photocatalytic activity. ► The WO3/Bi2WO6 is composed of WO3 nanoparticles and Bi2WO6 nanoplatelets. ► The heterojunction of two semiconductors is responsible for the enhanced activity.

Introduction

The application of solar energy for degradation of organic pollutants by semiconductor photocatalysts has received extensive attention [1], [2], [3], [4]. Sunlight contains 4% ultraviolet and 46% visible light, which can be used as excitation light source for semiconductor photocatalysts [5], [6]. Therefore, using sunlight to develop effective visible light-driven photocatalyst will be greatly beneficial for environmental remediation. It is well known that the high performance of photocatalysts mainly depends on such characteristics as surface area, band gap, crystallinity, quantum efficiency and so on [7], [8], [9]. Pure semiconductor oxides usually exhibit low photocatalytic activity because of high recombination probability of photogenerated electrons and holes. In order to cope with this problem, great efforts have been made on the improvement of photocatalytic activity. Besides doping by metal or non-metal ions and modifying by noble metals [10], [11], [12], fabrication of heterojunction photocatalysts containing two semiconductors with appropriate band edges has been a significant approach [13], [14], [15]. The interconnected interface of heterojunction composite results in low recombination ratio of photoinduced electron–hole pairs, thus high photocatalytic activity can be achieved.

Among all the semiconductor oxides, Bi2WO6 and WO3 as narrow band gap photocatalysts have attracted extensive attention. Bi2WO6 is an n-type semiconductor with a direct band gap of 2.8 eV and has suitable band edges (ECB = 0.46, EVB = 3.26 eV), which match well with WO3 (ECB = 0.4, EVB = 3.2 eV) to form a heterojunction photocatalyst [16], [17]. Because of the more negative conduction band (CB) level of WO3, electrons can be injected from the CB of WO3 to the CB of Bi2WO6 under visible irradiation. Thus, the photogenerated electrons and holes are separated in time. What is worth mentioning is that the synthesis procedure of most heterojunction catalysts is complex thus far. For example, all the three-dimensional ZnO/ZnSe, nanostructured WO3/BiVO4 and ZnO/CdS heterostructures have been synthesized by multi-step synthesis routes [18], [19], [20]. However, it is widely assumed that one-step method for preparation of heterojunction photocatalysts is more promising in terms of application.

In the present study, we first successfully prepared WO3/Bi2WO6 heterojunction photocatalysts comprising Bi2WO6 nanoplatelets decorated with WO3 nanoparticles by a facile hydrothermal reaction with excess amount of Na2WO4·2H2O according to the stoichimetric molar ratio of Bi/W = 2/1 and nitric acid. During the degradation of rhodamine B (RhB) under visible light irradiation, the WO3/Bi2WO6 photocatalyst exhibits much higher photocatalytic activity than that of the bare WO3 and Bi2WO6.

Section snippets

Experimental section

All chemicals are of reagent grade and were used without further purification. The detailed synthesis procedure is as follows: 0.97 g Bi(NO3)3·5H2O was firstly dissolved in 25 mL deionized water under ultrasonication for 10 min. 5 mL 65% nitric acid was dropped slowly into the above solution and stirring for 10 min. Subsequently, 5 mL aqueous solution containing 0.519 g Na2WO4·2H2O was added and a precipitate appeared with its color gradually turning from white to light yellow. The mixture was then

Results and discussion

The XRD patterns of Bi2WO6, WO3 and WO3/Bi2WO6 heterojunction photocatalysts are shown in Fig. 1. The pure Bi2WO6 and WO3 are orthorhombic (JCPDS NO. 39-0256) and monoclinic (JCPDS NO. 43-1035), respectively. The sample with molar ratio of Bi3+/W6+ = 2/1.58 shows intensified WO3 diffraction peaks, while the intensity of the peaks of Bi2WO6 decreases compared to the bare Bi2WO6 sample. No impurity peak is found in WO3/Bi2WO6 heterojunction photocatalyst. It is evident that the WO3/Bi2WO6 (Bi3+/W6+ =

Conclusion

The WO3/Bi2WO6 heterojunction photocatalysts were successfully synthesized by a one-step hydrothermal method. The as-prepared heterostructure catalysts are composed of WO3 nanoparticles grown on the 2D Bi2WO6 nanoplatelets. The morphology of the WO3/Bi2WO6 heterojunction is affected by the concentration of nitric acid and amount of Na2WO4·2H2O in the reaction solution. As an intermediate product, H2WO4 protected the inner Bi2WO6 from further acid corrosion. Under visible light irradiation, WO3

Acknowledgments

The authors thank the National Natural Science Foundation of China (No. 21043005) for financial support. Y.Q. Chang would also like to extend her thanks to the Student Research Program of SCUT for the financial aid.

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